How to use
- Tick Read, Write and Execute for the owner, the group and everyone else. Or press a common mode such as 755 or 644.
- You can also type an octal mode (755, or 4755 with a special bit) or the nine-character rwx form. The checkboxes and the other form update.
- Copy the octal value, the rwx value or the full chmod command. Change the file name to put it into the command.
Examples
755
→ rwxr-xr-xThe owner can read, write and execute. The group and others can read and execute. Typical for programs and folders.
644
→ rw-r--r--The owner can read and write. Everyone else can only read. Typical for ordinary files.
4755
→ rwsr-xr-xThe leading 4 sets the setuid bit, shown as s in place of the owner's x.
rwSr--r--
→ 4644A capital S means setuid is set but the owner's execute bit is off.
How it works
A Unix file mode has three groups of permission bits, for the owner, the group and everyone else, and an optional fourth group of special bits. Each group is a number from 0 to 7 made of read (4), write (2) and execute (1). The octal form writes those numbers one after another, so 755 is owner 7, group 5, others 5.
The rwx form is how ls -l shows the same mode: three letters per class, with - for a bit that is off. Setuid, setgid and sticky appear as s, s and t in the execute position of the owner, group and others.
The tool converts in both directions with plain arithmetic on these bits, and the whole round trip from 000 to 777 is covered by automated tests. It builds the command chmod <octal> <file>; it does not run anything and cannot change any file.
Sources
Frequently asked questions
- How do the octal digits work?
- Each digit is the sum of read (4), write (2) and execute (1) for one class. The first digit is the owner, the second the group and the third everyone else. So 7 is 4+2+1 (all three), 6 is 4+2 (read and write), 5 is 4+1 (read and execute) and 0 is nothing.
- What do setuid, setgid and sticky mean?
- Setuid makes a program run with the privileges of the file's owner. Setgid does the same for the group, and on a directory it makes new files inherit the directory's group. Sticky on a directory means only a file's owner, the directory's owner or root can delete or rename the files in it. The shared /tmp folder uses 1777.
- What do the capital S and T mean?
- In the rwx form, s and t replace x when the special bit and the execute bit are both on. A capital S or T means the special bit is on but execute is off, which is rarely what you want.
- Is 777 a good idea?
- Usually not. 777 lets every user on the system change and run the file, which is a security risk and often hides the real permission problem. Prefer the narrowest mode that works, such as 755 for programs and 644 for files.
- Is anything sent to a server?
- No. The conversion runs in your browser, and the page never sees your files.